Based on recent surveys in Illinois, bean pod mottle virus (BPMV) appears to be the state's most common soybean virus. BPMV was first reported in Illinois in 1975 and now occurs across the state. Many soybean producers, especially those involved with food-grade soybean and seed production, are concerned about this virus and the effects it may have on crop quality and yield in 2003. In the May 16, 2003, edition of the Bulletin, Kevin Steffey (UIUC Extension entomologist) wrote about bean leaf beetles and briefly discussed BPMV in relation to this insect pest. This article summarizes selected key information on BPMV and its vectors.
Several different symptoms are associated with infection of soybean by BPMV. Common symptoms are green to yellow mottling of leaves, and severe strains of the virus may cause puckering and distortion of leaves. These symptoms may be masked by high temperatures and not seen after pod set. BPMV infection is associated with seed coat mottling and green stem syndrome. BPMV can also cause reduced seed size, weight, and number, and can predispose plants to infection by Phomopsis (a fungal pathogen).
It is important to keep in mind that these symptoms can also have other causes. Leaf symptoms can be similar to some types of nutrient deficiency and injury caused by benzoic acid or phenoxy herbicides. The similarity between some virus symptoms and herbicide injury can be confusing; the true problem can best be determined by careful diagnosis and by observing the pattern of symptoms in a field and the timing and history of herbicide applications.
The relationship between seed mottling and green stem syndrome is also complex. Based on recent research from Illinois, BPMV has been associated with these symptoms, but it is often not a cause-and-effect relationship. Seeds and plants that have mottling or green stem syndrome often do not have detectable levels of the virus, and vice versaseeds and plants with BPMV often show no symptoms at all. The symptoms resulting from virus infection can vary depending on environmental conditions, time of infection, strain of virus, and soybean variety. Laboratory diagnosis is required to confirm whether or not plants or seeds are infected with BPMV. Contact the University of Illinois Plant Clinic for information on testing for BPMV (217-333-0519 or e-mail firstname.lastname@example.org).
BPMV is transmitted by beetles (bean leaf beetle and western corn rootworm) and infected seed. Studies have indicated that the rate of transmission of BPMV through seed is low (0.1%), but even at this low rate, the number of infected plants per acre may be high enough to initiate problems if vectors are present to spread the virus. The bean leaf beetle may be the most important vector over much of the state of Illinois and in other nearby states. This insect pest can harbor the virus over the winter or acquire it by feeding on infected soybeans or other host plants in the spring.
In addition, the western corn rootworm also may be an important vector in Illinois. Recent research in Illinois by the Illinois Natural History Survey, University of Illinois, and USDA scientists has shown for the first time that the western corn rootworm beetles are able to transmit BPMV to soybean. The presence of BPMV in western corn rootworm has been confirmed in at least 20 counties in east-central Illinois. It is important to note that the transmission efficiency of BPMV was lower for the western corn rootworm than for the bean leaf beetle, but the western corn rootworm has greater mobility and could result in increased spread of BPMV.
Management of BPMV is a challenge. The following tactics may be useful but are largely unproven: delay planting to avoid overwintering beetles, plant virus-free seed, control perennial weeds that may be alternate hosts for the virus, and in some cases insecticides may be warranted. At this point, the use of insecticides (either foliar sprays or seed treatments) in Illinois to control BPMV cannot be justified based on available research data. Some possible exceptions may be in areas where BPMV has been verified to cause significant yield loss or mottling of seed, and near woods, alfalfa fields, and other areas where large numbers of bean leaf beetles have been observed. Remember that even if all bean leaf beetles are controlled (an impossible achievement), infection via seed or western corn rootworms may also occur. Commercial soybean cultivars with resistance to BPMV are not available, although there appear to be differences in susceptibility and expression of symptoms among some cultivars. Many questions remain to be answered, and research is under way to develop answers to some of the critical questions.--Dean Malvick